This invention relates to a non-aqueous electrolyte for fabricating a two-terminal nonlinear element that is used as a switching element, a method of fabricating a two-terminal nonlinear element by using this non-aqueous electrolyte, a two-terminal nonlinear element obtained by this fabrication method, and a liquid crystal display panel using that element.
In an active matrix type of liquid crystal display device, a liquid crystal fills a space between an active matrix substrate, which is formed as a matrix array with a switching element provided for each pixel region, and a facing substrate provided with a color filter or the like. Predetermined image information can be displayed by controlling the alignment of the liquid crystal in each pixel region. In general, a three-terminal element such as a thin-film transistor (TFT) or a two-terminal element such as a metal-insulator-metal (MIM) type of nonlinear element (hereinafter called a xe2x80x9cMIM elementxe2x80x9d) is used as each of these switching elements. A switching element using a two-terminal element is considered to be better than a three-terminal element in that there is no cross-over shorting and the fabrication thereof can be simplified.
To implement a liquid crystal display panel with a high image quality that has good contrast, and also no discernible display unevenness, after-image, or image persistence within a liquid crystal display device using MIM elements, it is important to ensure that the characteristics of the MIM elements satisfy the following conditions:
(1) The capacitance of each MIM element must be sufficiently smaller than the capacitance of the liquid crystal display panel,
(2) Changes with time in the voltage-current characteristic of the MIM element must be sufficiently small,
(3) The symmetry of the voltage-current characteristic of the MIM element must be good,
(4) The steepness of the voltage-current characteristic of the MIM element must be sufficiently high, and
(5) The resistance of the MIM element must be sufficiently uniform over a wide area.
In other words, to increase the contrast, it is necessary to make the capacitance of the MIM component sufficiently small with respect to the capacitance of the liquid crystal display panel, and also ensure that the steepness of the voltage-current characteristic of the MIM component is sufficiently large. To ensure there is no discernible display unevenness, it is necessary to make the resistance of the MIM component sufficiently uniform over a wide area. To ensure there is no discernible after-image, it is necessary to make sure that the changes with time in the voltage-current characteristic of the MIM element are sufficiently small. Furthermore, to ensure there is no discernible image sticking, it is necessary to make sure that the changes with time in the voltage-current characteristic of the MIM element are sufficiently small, and also that the symmetry of the voltage-current characteristic of the MIM element is good.
In this case, xe2x80x9cafter-imagexe2x80x9d is a phenomenon that occurs when another image is displayed after a certain image has been displayed for several minutes, in which case the previous image can still be discerned. Similarly, xe2x80x9cimage stickingxe2x80x9d is a phenomenon that occurs when another image is displayed after a certain image has been displayed for several hours, in which case the previous image can still be discerned. The phrase xe2x80x9cthe symmetry of the voltage-current characteristic is goodxe2x80x9d means that, when a current flows from the first conductive film to the second conductive film under a certain voltage and when a current flows from the second conductive film to the first conductive film, the difference in absolute values of these currents is sufficiently small.
Examples of documents that disclose techniques for MIM elements are listed below.
(a) Japanese Patent Application Laid-Open No. 52-149090 discloses an MIM element that is fabricated from a first conductive film of tantalum, an insulated film that is a metal oxide film formed by anodic oxidation of this first conductive film, and a second conductive film of chromium formed on a surface of this insulated film. The insulated film is formed to a uniform thickness without pinholes, by forming it by anodic oxidation of the surface of the first conductive film. Japanese Patent Application Laid-Open No. 57-122478 disclosed the use of a dilute aqueous solution of citric acid as the electrolyte for anodic oxidation.
These techniques do not necessarily ensure sufficient quality for the above characteristics (2) to (5) of the resultant MIM element. In other words, they are unsatisfactory from the viewpoints of changes with time, symmetry, and steepness of the voltage-current characteristic, and also the resistance of the element is not sufficiently uniform over a wide area. This means that it would be difficult to ensure a high level of contrast over a wide temperature range in a liquid crystal display device using such MIM elements, and there will be problems such as a tendency towards unevenness in the display.
(b) The international application PCT/JP94/00204 (International Publication No. WO94/18600) discloses a configuration in which is used a film of an alloy of tantalum to which tungsten is added, as the first conductive film of the MIM element.
Since the first conductive film of the MIM element produced by this technique is a film of an alloy comprising tantalum and a specific element such as tungsten, instead of tantalum alone, this provides an improvement over the techniques disclosed in the documents of (a) with respect to characteristics (2) and (3), in other words, the changes with time and the symmetry of the voltage-current characteristic of the MIM element, so it is capable of improving quality to a level at which after-images cannot be discerned, and also of maintaining a good contrast over a wide temperature range. However, this technique has a problem concerning insufficient margin in the contrast characteristics required of such an element at high temperatures.
(c) Jpn. J. Appl. Phys, 31,4582 (1992) discloses the use of a dilute aqueous solution of phosphoric acid or ammonium borate as the electrolyte for the anodic oxidation used for forming the insulated film of an MIM element.
This technique provides an improvement over the techniques disclosed in the documents of (a) with respect to characteristics (2) and (3), in other words, the changes with time and the symmetry of the voltage-current characteristic of the MIM element, so it is capable of improving quality to a level at which after-images cannot be discerned, and also of maintaining a good contrast over a wide temperature range. However, this also has problems in that the reliability of the resultant elements is low, they are likely to be destroyed by short-circuiting, and display unevenness easily occurs.
(d) Japanese Patent Application Laid-Open No. 2-93433 discloses a configuration in which a film of an alloy of tantalum and silicon is used as the first conductive film of the MIM element.
This technique made it possible to improve the steepness of the voltage-current characteristic in comparison with the techniques of the documents of (a), and also provide sufficient margin over a wide temperature range to ensure a high contrast. However, this technique also has problems in that the reliability of the element is low and thus it can easily be destroyed, and display unevenness can easily occur.
An objective of this invention is to provide a two-terminal nonlinear element that satisfies all the characteristics (1) to (5) required of the above described MIM element, particularly a capacitance that is sufficiently small, a voltage-current characteristic with a sufficiently large steepness, and a resistance that is sufficiently uniform over a wide area; and also a liquid crystal display panel that uses this two-terminal nonlinear element and has a high image quality with a good contrast and no display unevenness.
Another objective of this invention is to provide a non-aqueous electrolyte for fabricating a two-terminal nonlinear element having the above described superior characteristics, as well as a fabrication method using that electrolyte.
As used herein, the term xe2x80x9cnon-aqueousxe2x80x9d refers to the primary character of the electrolyte, meaning that the electrolyte is not an aqueous electrolyte comprising a substantial portion of water. That is, the xe2x80x9cnon-aqueousxe2x80x9d electrolyte comprises an organic solvent and solute. The electrolyte can also include water, in an amount of from 1 to 10 wt %, while still being a xe2x80x9cnon-aqueousxe2x80x9d electrolyte.
The non-aqueous electrolyte for fabricating a two-terminal nonlinear element (hereinafter called a xe2x80x9cMIM nonlinear elementxe2x80x9d) in accordance with a first aspect of this invention comprises an organic solvent and solute, and also has an electrolytic conductivity that is greater than or equal to 1 mS/cm but less than or equal to 100 mS/cm, but is preferably greater than or equal to 1 mS/cm but less than or equal to 10 mS/cm.
Using this electrolyte to perform anodic oxidation on the first conductive film, which is formed on the substrate of tantalum or a tantalum alloy, makes it possible to form an oxide film which has a uniform film quality and which also has a thickness that is sufficiently uniform over the entire surface of the substrate. Therefore, an MIM nonlinear element obtained by anodic oxidation using this electrolyte has a resistance that is sufficiently uniform over a wide area. In an MIM nonlinear element in accordance with this invention, the material of the second conductive film is not limited to a metal; the definition thereof also comprises a conductive film of a material such as indium tin oxide (ITO).
The permeation of the solute or solvent, or both solute and solvent, into the oxide film during the anodic oxidation makes it possible to reduce the relative permittivity of the oxide film (insulated film) to within a suitable range. As a result, the thus obtained MIM nonlinear element has a capacitance that is sufficiently small, and also the steepness of the voltage-current characteristic thereof is high.
The solute may comprise at least one of a carboxylate and a salt of inorganic oxoacid.
This carboxylate may be at least one salt of carboxylic acids selected from the group of aromatic carboxylic acids and aliphatic dicarboxylic acids having no hydroxyl groups. This carboxylate is preferably an aromatic carboxylate, and at least one of salicylate and phthalate is particularly preferable.
The central atom of the oxoacid in the salt of inorganic oxoacid may be an atom belonging to one of Groups IIIA, IIIB, IVA, IVB, VA, VB, VIA, VIB, VIIA, and VIIB (CAS version) of the periodic table. In addition, this salt of inorganic oxoacid may be at least one type selected from the group of nitrates, vanadates, phosphates, chromates, tungstates, molybdates, silicates, perrhenates, borates, and sulfates. A tungstate is preferable as this salt of inorganic oxoacid, and at least one type of primary, secondary, tertiary, and quaternary ammonium salt is particularly preferable.
The non-aqueous electrolyte of this invention comprises an organic solvent and a solute, and it may also comprise water where the proportion thereof with respect to the electrolyte is preferably 1 to 10 wt %.
This organic solvent may be at least one of ethylene glycol and xcex3-butyrolactone.
An MIM nonlinear element in accordance with a second aspect of this invention comprises a first conductive film, insulated film, and second conductive film deposited in sequence on a substrate, wherein the first conductive film is of tantalum or a tantalum alloy; and wherein the insulated film is formed by anodic oxidation of the first conductive film, comprises carbon atoms or at least one type of element belonging to at least one of Groups IIIA, IIIB, IVA, IVB, VA, VB, VIA, VIB, VIIA, and VIIB of the periodic table and originating from the central atom of an inorganic oxoacid, and also has a relative permittivity of 10 to 25. The relative permittivity of the insulated film may be more preferably 22 to 25, to ensure sufficiently small changes with time in the MIM nonlinear element.
The carbon atoms or at least one type of element belonging to at least one of families 3 to 7 of the periodic table and originating from the central atom of an inorganic oxoacid, comprised within the insulated film, may be distributed through the entire thickness direction of the insulated film.
The relative intensity of these carbon atoms with respect to oxygen atoms (18O) may be 0.2 to 1000 throughout the entire thickness direction of the insulated film, and more preferably 0.2 to 100, as determined by elemental analysis obtained by secondary ion mass spectrometry (SIMS) by irradiation of cesium primary ions.
The relative intensity of this at least one element in the insulated film belonging to Groups IIIA, IIIB, IVA, IVB, VA, VB, VIA, VIB, VIIA, and VIIB; of the periodic table may be at least 10 times the intensity of the element in the first conductive film, as determined by elemental analysis obtained by SIMS.
The insulated film of the MIM nonlinear element in accordance with this invention is obtained by anodic oxidation of the first conductive film in the non-aqueous electrolyte of this invention. The relative permittivity of the insulated film can be reduced to a suitable range by the permeation of the solute or solvent, or both the solute and solvent, into the oxide film during the anodic oxidation, which causes at least carbon atoms or at least one element belonging to Groups IIIA, IIIB, IVA, IVB, VA, VB, VIA, VIB, VIIA, and VIIB of the periodic table and originally the central atoms of an inorganic oxoacid to become incorporated into the oxide film (insulated film). As a result, the MIM nonlinear element of this invention has superior characteristics such as a low capacitance and a large steepness of the voltage-current characteristic thereof.
A liquid crystal display panel in accordance with a third aspect of this invention includes the above described MIM nonlinear element. More specifically, it comprises a first substrate provided with a transparent substrate, one type of signal line disposed in a predetermined pattern on the substrate, a MIM nonlinear element in accordance with this invention, connected at a predetermined pitch to this signal line, and a pixel electrode connected to the MIM nonlinear element; a second substrate provided with another type of signal line positioned opposite to the pixel electrode; and a liquid crystal layer interposed between the first substrate and the second substrate. This liquid crystal display panel has a good contrast, is not likely to develop problems such as display unevenness, and therefore makes it possible to provide a high-quality image display such that it can be applied to a wide range of applications.